Date: Thu, 16 Jul 2020 11:56:29 -0700 From: John-Mark Gurney <jmg@funkthat.com> To: Marko Zec <zec@fer.hr> Cc: freebsd-net@FreeBSD.org Subject: Re: IF_DRV_PREPEND unlocked? Message-ID: <20200716185629.GT4213@funkthat.com> In-Reply-To: <20200716074917.04445daa@x23> References: <20200715232624.GR4213@funkthat.com> <20200716072622.5fa35ba2@x23> <20200716074917.04445daa@x23>
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Marko Zec wrote this message on Thu, Jul 16, 2020 at 07:49 +0200:
> On Thu, 16 Jul 2020 07:26:22 +0200
> Marko Zec <zec@fer.hr> wrote:
>
> > On Wed, 15 Jul 2020 16:26:25 -0700
> > John-Mark Gurney <jmg@funkthat.com> wrote:
> >
> > > I happen to be looking at the implementation of IFQ_DRV_PREPEND, and
> > > unlike IFQ_DRV_DEQUEUE, it doesn't obtain the lock when manipulating
> > > the ifq. I took a brief look at a few drivers, and it looks like
> > > some of them expect that _PREPEND lock the Q like _DEQUEUE does.
> > >
> > > This is likely not an issue often, since it's an error path that
> > > likely rarely happens, but we should fix it.
> > >
> > > Should we just add the IFQ_LOCK/_UNLOCK to the macro?
> > >
> > > Comments or thoughts?
> >
> > I also have a hard time understanding the semantics of IFQ_DRV_*
> > macros: per altq(9) they appear to be variants of their IFQ_
> > counterparts which are intended to be protected by some other
> > mechanism rather than IFQ_LOCK, but in some instances this isn't the
> > case, as they do grab the IFQ_LOCK.
>
> Hmm it's not that unclear after looking better: ifq_drv_* parts of
> struct ifaltq should be protected by some external mechanisms when
> using IFQ_DRV_ macros, and ifq_ parts are still protected by ifq_mtx.
> So it doesn't look like IFQ_LOCK/_UNLOCK should be added to
> IFQ_DRV_PREPEND()...
Ahh, I was miss reading the macros on what was protected. Yes, it does
look like the ifq_drv_* fields are supposed to be protected by another
lock...
I just now found that altq(9) documents this interface... I had assumed
it as undocumented after man IFQ_DRV_DEQUEUE didn't return results..
Looks like I REALLY do need to complete my script to make sure that ALL
Nm's and related pages have MLINKs. ALTQ.9 is missing 16 links to the
functions it documents...
Just as a reminder to people who write docs, please, make sure you link
ALL of the function in your docs... Otherwise your docs won't be as
easy to find..
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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